It's not clear. The first reference was to changing bulbs, and then it's changing a bayonet socket. I'm not sure why the rear gate (hatch) trim (if that's what's meant by "plastic back") has to come off in order to do this, as both the bulbs and the sockets are supposed to be accessible through smaller snap-off hatches in the rear gate trim. However, if the rear gate wiring harness is being replaced, then the trim does have to come off.
The dealer certainly has access to the service documentation that includes how to remove the trim, so the lack of knowledge in this regard is also strange.
I'll also add that I thought Subaru no longer uses bayonet-type bulbs for the tail and stop lights. Must be a misunderstanding, but I think we'd be better able to help if there we have clearer information.